If you want fine positioning they're pretty good (about 4000 half-steps per revolution, so < 0.1 degree), although there are a few degrees of play (backlash) on the shaft, so some accommodation would be needed to address that. The spec sheet gives a max 1000pps at no load, so that amounts to about 15rpm max.

I'd like to build a robot that would wonder the room avoiding obstacles. Nothing fancy.

You need to work out some sort of size / weight as a starting point - everything follows from that - a tinymicro robot could use the 28YBJ's, but would probably have Arduino Mini, 3 AAA's for batteries, and bevery "light duty". I was possibly a little dismissive of them in retrospect.

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I've been experimenting with the 28BYJ-48 5V stepper to see what kind of application it might fit into--other than operating air conditioner louvers!

The torque is not very large. In a rough measurement I get about 250 g-cm. That's using an Arduino Uno to drive it, powered by USB, with the ULN2003 driver board that comes with the motor. The spec sheet says >300 g-cm, so it's in the ballpark.

The torque is unsurprisingly strongly dependent on the supply voltage to the driver board. With USB providing power, I see on a scope that the voltage drops from 5.0V to about 4.2V when the coils are energized, so it probably loses a little there. I tried driving it with 6.0V from another supply and the torque about doubled, to ~500 g-cm, but at 6V the motor gets pretty hot after about a minute or so, so that's probably not recommended, unless the use is very intermittent.

It's probably best to connect a separate regulated power supply to the driver board rather than using the Arduino-regulated +5V or USB.

Terry has designed broadcast stations, recording studios, broadcast equipment, intelligent machines and special computer languages for IBM, and has worked as a broadcast journalist covering elections, fires, riots and Woodstock.
He has taught electronics

Those motors can be used for a robot, but it will be a slow robot... I've done it

http://www.youtube.com/watch?v=aLozZ_RYIRE

I was using a 3S Lipo (+-12V), makes the motors run pretty hot. I haven't killed them yet. They are dirt cheap so can take the risk I did need to write code to slowly ramp up the speed else I could not let them run that fast. And I used 2 74HC595 shift registers to control the 4 ULN2003 boards. So I only needed 3 pins to control the 4 steppers